1. Field of Invention
This invention discloses an optical performance improvement for optical fiber array minerals such as halochtrite and ulexite. These minerals are examples of natural fiber arrays that form an array of parallel fibers capable of transferring light and an image from one surface to a second surface. However, because of high crosstalk between the fibers, the minerals are currently not practical for many applications, and remain a scientific curiosity. Fiber crosstalk reduces the image sharpness, contrast and signal. This invention enhances mineral fiber array performance by greatly reducing fiber crosstalk between fibers, providing a sharper image with higher contrast, improved thruput and in some cases wider bandwidth.
2. Description of Related Art
A artificial equivalent to the mineral fiber arrays is an artificially manufactured optic referred to as faceplates, which are used to translate an image between two surfaces. These faceplates have a high index of refraction difference between the center of the optical fiber and the cladding, allowing for a numerical aperture of one. The main application has been as a window over a detector array, allowing the focal surface to be at the outside of the window. The thin window does not introduce high image spatial distortion caused by non parallel fibers. Thicker artificial fiber arrays are subject to higher image distortion because of the non parallel fibers.
Usable fiber optic faceplates currently are artificially constructed by forming optical fibers in a parallel manner, fusing the assemblage, then drawing the melted assemblage to reduce fiber size. The index of refraction difference between the core and the cladding is high enough to yield a numerical aperture of one, which eliminates fiber crosstalk. A disadvantage is that the process does not maintain the fibers parallel, which introduces spatial distortion.
In summary, the main disadvantages of the artificially manufactured optics are cost and spatial distortion. The longer the component's length, the greater the image distortion, which limits the applications. An example of the artificially manufactured fiber optic component, referred to as a faceplate, is Schott fiber optic faceplate 47A. The faceplate terminology describes the use and short length. The short length minimizes spatial distortion.
Natural minerals, for example from the halochtrite family, also exhibit fiber optic array characteristics. The best know is ulexite. These mineral fiber arrays, such as ulexite, have a crystal that exhibits high micro fracturing, forming parallel fibers with a micro separation between the parallel micro fiber crystal segments. The parallel fiber segments acting together translate an image from one end of the crystal fiber array to the other end; however, in the case of ulexite, there is considerable fiber crosstalk resulting in contrast reduction, image smearing and significant signal loss. Ulexite is commonly called TV rock in recognition of this image translation phenomenon. Many minerals exhibit the image translation phenomenon, and can be synthetically grown. The naturally occurring minerals typically have impurities such as clay which detracts from performance. Synthetically grown natural minerals can avoid these impurities, and possibly add materials to enhance the desired properties.
A study of ulexite was reported in a paper by Subrata Ghose and an titled “Ulexite, NaCaBO6(OH)6.5H2O: Structure Refinement, Polyanion Configuration, Hydrogen Bonding, and Fiber Optics” published in American Mineralogist, Volume 63, pages 160-171, (1978), which describes the nature of ulexite wherein the mineral forms spaces between adjacent parallel fiber crystals. The crystals are described as parallel crystals that act as fibers and are not packed solidly, having spaces on the order of 0.5 micrometers or less surrounding each fiber crystal. These spaces create an index of refraction drop at the edge of each fiber crystal causing partial light reflection off of the fiber crystal boundary and thus channel the light along the crystal fiber. The phenomenon supports an image translation between opposite faces perpendicular to the crystal fibers. Subrata Ghose also identifies that the spaces between the crystals support capillary action wherein a colored liquid enters the spaces between the crystals and travels along the space between the crystal fibers. Subrata Ghose reports no change in optical characteristics because of a liquid introduced into the spaces. Subrata Ghose did not identify the possibility of introducing materials that indeed would improve the fiber optic performance. A photograph of the crystals is provided in the paper, which shows the crystal structure pattern very similar to the cracked mud in a dry lake bottom. Subrata Ghose does not speculate how the fibers are formed.
Subrata Ghose states the possibility that the spaces between the crystals may be artificially arranged to eliminate the random cross sectional array and maximize the core to cladding index difference, but no method is advanced. Eliminating the cracking randomness would have the benefit of forming fiber shapes and sizes to better translate an image by matching the wavelength and detector array pixel geometry.
The fibers for ulexite are in close proximity, 0.2 to 0.5 microns separation as described by Subrata Ghose. The close proximity between fibers and low index of refraction difference between the fiber and the spaces cause considerable mode locking between adjacent fibers resulting in severe crosstalk. The index of refraction difference at the surface of the fibers varies from zero to 0.3 causing even higher fiber coupling at the low index of refraction locations. Fiber mode locking is a function of the near proximity between fibers as well as fiber shape. The natural difference in index of refraction between the fiber crystal and the fiber spaces are identified as a maximum of 0.3, which is insufficient to support a usable fiber optic array with close fiber packing.
No discussion in the literature is discovered, but my speculation is that the crystal fibers are formed as the ulexite mineral shrinks upon water loss, causing a complex cracking pattern, but maintaining geometrical shapes and forming random interlocking geometric crystal shapes. The process appears to be similar to the dry lakebed cracking pattern.
The optical characteristics of tapered gold coated optical fibers is reported by A. Hartung, F. Wirth, and H. Bartelt in a paper titled “Light Propagation in Tapered Optical Fibers: Spatial Light Confinement and Generation of Plasmonic Waves” published in Progress In Electromagnetics Research Symposium Proceedings, Marrakesh, Morocco, page 255 (Mar. 20-23, 2011). The paper reports that tapered gold coated fiber's light propagation in an optical waveguides as fundamentally different from free space propagation because light spreading by diffraction is avoided. The light is confined to modes which stay constant in shape along perfect waveguides and which can propagate with low attenuation for long propagation lengths. The metallic coated fibers support light propagation of a quarter of the wavelength, which is not true for typical fibers using a low index cladding. The geometrical fiber shape being very critical to performance.
The Hartung paper does not discuss mineral optical fiber arrays; however, this observation suggests that natural mineral fiber arrays with reflective interfaces may support longer wavelengths such as MWIR. The Hartung paper does not report on an advantage of gold coated fibers in an optical array wherein the fibers are in close proximity.
U.S. Pat. No. 5,061,035 A by Harvey N. Rogers, Jr. teaches use of metal coated fiber arrays including gold as a method to hermetically seal optical fiber arrays. The invention is comprised of a bundle of metal coated optical fibers which are sealed to each other and to a metal coated supporting structure. The application is for artificially manufactured fiber arrays and is not considered as a method to reduce crosstalk in a fiber array.
A paper by Nobuyuki Takeyasu, Takuo Tanaka and Satoshi Kawata, titled, “Metal Deposition Deep into Microsturcture by Electroless Plating” published in the Japanese Journal of Applied Physics, Vol. 44, No. 35, pp. L 1134-L 1137, (2005), describes electroless plating inside fine structures. The paper discusses gold deposition inside a capillary tube and a complex concave structure of micrometer scale consisting of polystyrene microbeads sandwiched between glass plates. The paper uses a complicated chemical process to deposit gold similar to silver plating on mirrors. A simpler method to deposit gold can be achieved using gold trichloride, with the gold precipitating when exposed to light as currently performed in photography.
U.S. Pat. No. 3,797,910 by Ralph A. Westwig, uses artificially made optical fibers laid parallel and fused as an imaging translation optic. Westwig teaches having every other optical square fiber constructed with a transparent cladding and an absorbent cladding. The absorbent cladding absorbs the vertically Polarized light, reducing crosstalk. Westwig's prior art teaches making a artificial optical fibers by forming a low index cladding around a high index core. The reflections at the interface of the core and cladding channel the light along the optical fiber. The cladding's second purpose is a method to separate the fiber cores and reduce fiber mode locking. Westwig does not use mineral fibers or utilize natural minerals such as ulexite that perform an imaging translation phenomenon. Therefore Westwig does not have the same fiber structure, and lacks the fractures between crystal fibers. Westwig cannot teach filling the fracture spaces between the fibers as a method to reduce crosstalk between fibers. Westwig does not teach use of optically active materials external to his fiber such as gold exterior to the cladding as a method to reduce mode locking.
Westwig, teaches a method to reduce fiber crosstalk by adding an absorbent cladding to every other optical fiber's cladding. Westwig, in accordance with the prior art uses a artificial formed optical fiber with a high index core and a low index cladding. However, the index of refraction difference between the core and cladding is not large enough to reflect the vertically polarized light allowing the vertically polarized light to enter the adjacent fiber in a cross coupling mode. Westig refers to the vertically polarized light as second light. To overcome the dilemma, Westwig teaches a construction wherein a second fiber is constructed of a absorbent cladding rather than trasmissive cladding, thus absorbing the vertically polarized light. The absorbent cladding has the disadvantage of reducing the light throughput greater than fifty percent. Westwig did not consider reflective coatings outside of the cladding which is another method to reduce crosstalk, but has the further advantage of transmitting both horizontal and vertically polarized light.
Another disadvantage of the Westwig design is that all fibers are the same size, square, and in a uniform pattern in lines. This construction is the worst possible configuration because it causes severe Moiré patterns when used with a detector array with similar configuration. The fiber arrangement beats with the detector pixel array arrangement, creating severe Moiré Patterns. In order to avoid these Moiré patterns, Westwig must match each fiber with each pixel on the detector array, a very laborious, difficult task that in practice is expensive. Westig did not consider different geometric shaped fibers of different sizes, which is a method to avoid Moiré patterns.
The methods required to fabricate Westwig's design is labor intensive because of the many steps and accuracy required to lay each fiber independently. The required accuracy to maintain the fibers parallel through the fabrication process is typically not achieved. Current optics fabricated similar to the prior art used by Westwig does not achieve parallel fibers over any distance, limiting their use to thin sections rather than longer image translation devices. Westwig could not consider natural minerals such as ulexite that provide an assembled optic with parallel crystal fibers because the ulexite is of a totally different structure, and violates Westwig's classical construction of optical fibers with high index core and low index cladding. The crystal like parallel needles act as fibers but do not have a cladding. The small spaces between the mineral crystal fibers act similar to cladding; however, exhibit high cross coupling because of near positioning and low index difference between the edge of the crystal fiber and fractured open space between crystal fibers.
Crystalline minerals also exhibit birefringence causing the desirable parallel polarized light to be converted to vertically polarized light which increases the cross coupling to the adjacent fiber. Thus Westwig's basic principle of absorbing the vertically Polarized light in the second fibers cladding is not supported by any birefringent core, restricting the core to only man made amorphous materials.